An electrophotographic apparatus is used for recording picture image data on an electrophotographic microfilm in the form of an electrophotosensitive member which is formed by laminating a layer of a transparent conductive material and a layer of a transparent photoconductive insulating material on a base of a transparent insulating material. The principle of recording picture image data on such a film by the electrophotographic apparatus includes the steps of uniformly electrostatically charging the film, exposing picture image data carried by an original onto the charged film to leave a pattern of electrical charges corresponding to the picture image data on the film, developing the residual charge pattern on the film with a charged toner, and finally fixing the toner image on the film. Thus, the picture image data are recorded by the processing steps of charging, exposure, development and fixing. Unlike a conventional silver-halide photographic film, the electrophotographic film used in this electrophotographic apparatus is featured in that it does not become photosensitive until it is electrostatically charged. Therefore, in the case of the electrophotographic apparatus, additional recording of new picture image data on non-recorded frames of the film can be very easily achieved by merely electrostatically charging the film immediately before the step of exposure as described above. In contrast, in the case of a conventional photographic apparatus using a silver-halide photographic film, additional recording of new picture image data on the film is almost impossible due to the difficulty of complete shielding of the film against light although this complete shielding is required at the time of film exchange, during storage and, especially, at the time of data retrieval before projection.
However, the charges provided by electrostatically charging the film used in the electrophotographic apparatus are lost by natural extinction in a very short period of time, and, therefore it is necessary to perform the steps of exposure and development before the charges on the film disappear. Thus, there is the requirement that the steps of charging, exposure and development should be completed within a short length of time. As a means of satisfying this requirement, a process head including a charging and exposing section, a developing section and a fixing section arranged in continuous series has been proposed so that the steps of charging and exposure, development and fixing can be simultaneously performed by the individual sections respectively of the process head on an electrophotographic film moving along the process head.
A practical form of such a process head will be described with reference to FIG. 6. FIG. 6 shows a charging and exposing chamber 1 and a developing chamber 2 only of the process head. Referring to FIG. 6, a corona discharge electrode 4 disposed in the charging and exposing chamber 1 radiates corona ions uniformly toward and onto an electrophotographic film 3 for uniformly electrostatically charging the film 3, and light condensed by a lens 5 is focused on the film 3 to expose the film 3 at the same position thereby forming a residual charge pattern corresponding to picture image data carried by an original. Subsequently, the film 3 is moved by a distance corresponding to one frame to bring the exposed frame into engagement with the developing chamber 2. In the developing chamber 2, a charged toner disposed in a liquid developer is supplied toward and onto the surface of the exposed frame of the film 3 thereby developing the residual charge pattern. In the process head shown in FIG. 6, a film retaining member 6 in the form of a flat plate is disposed for pressing the film 3 against the charging and exposing chamber 1 and the developing chamber 2 of the process head, so that the portion of the film 3 brought opposite to the charging and exposing chamber 1 can maintain the desired positional accuracy relative to the process head and is maintained as flat as possible and also so that the liquid developer may not externally leak from the developing chamber 2 through the portion of the film 3 brought opposite to the developing chamber 2. A condenser lens 7 and a projection lamp 8 for projecting the picture image data recorded already on the film 3 are disposed on the other side of the film retaining member 6, that is, on the side remote from the exposure lens 5 of the charging and exposing chamber 1. In an electrophotographic apparatus including the process head having such a structure, the charging and exposing chamber 1 has the functions of charging, exposure and projection. The electrophotographic process head of the above type is described in detail in, for example, Japanese unexamined Patent Publication No. 59-100479(1984). Also, an apparatus using a process head similar to that described above for carrying out, at the same position, the charging and exposing operation and the projecting operation projecting recorded picture image data is disclosed in the applicant's earlier applications, for example, Japanese Patent Applications Nos. 58-84079, 58-173869 and 58-173870 (1983).
In the process head of the aforementioned structure, the charging and exposure of a film and the projection of recorded picture image data are carried out in the same chamber. Therefore the entire film cannot be supported by a member such as a glass plate. As a result, in the projection mode in which the luminous intensity of projection light is as high as about 100,000 luxes, the film cannot maintain its flatness due to thermal deformation attributable to the temperature rise, and out of focusing results necessarily. The desired flatness of the film may be maintained when a vacuum-sucked plate is employed to serve as the film retaining member 6. However, employment of such a plate results in a bulky overall size of the apparatus, and the apparatus becomes quite expensive. Also, when the film is to be loaded in a cassette, the vacuum-sucked plate must also be incorporated in the cassette, resulting in a bulky overall size of the cassette. Further, the desired flatness of the film may be maintained when a projection chamber is separately provided. However, this results in the loss of the desired compactness of the process head.